SUBMARINE LANDSLIDES IN SOCIETY AND AUSTRAL ISLANDS,
FRENCH POLYNESIA: EVOLUTION WITH THE AGE OF EDIFICES
V. CLOUARD
Departamento de Geofísica, Univ. de Chile, Blanco Encalada, 2085, Casilla 2777,
Santiago, Chile
A. BONNEVILLE
Géosciences Marines, CNRS, Institut de Physique du Globe, 4 place Jussieu, 75252
Paris Cedex 05, France
Abstract
This paper describes numerous submarine landslides in French Polynesia. This
inventory shows an evolution of the landslide type with oceanic islands age. Submarine
active volcanoes are subject to superficial landslides of fragmental material whereas
young islands exhibit marks of mass wasting corresponding to giant lateral collapses
producing debris avalanche during the period of volcanic activity. Later, erosional
processes generate fine grained debris avalanches and carving the stellate morphology
observed on atolls and guyots. In addition, the shape of Tupai atoll and Rurutu Island
have been deeply modified by giant slumps that deeply modify their shape.
Keywords: Submarine landslide, French Polynesia, Austral, Society,
classification,oceanic islands
1. Introduction
Landslides are common features of oceanic island and play a key role in their
evolution. Due to caldera collapse or flank collapses, they can be of three types : (1)
rock falls, (2) slumps or (3) debris avalanche (Moore et al., 1989). Rock falls or
superficial landslides are mainly related to erosional processes of the subaerial parts of
the island, debris are less than 1 m in size, and their surface is rippled. Flank collapses
generally produce giant submarine landslides, with a amphitheatre at their head (Moore
et al., 1989). Landslides due to a deep listric fault are cataclysmic events producing fast
moving debris avalanche. Deposits can stretch out on several hundreds kilometers
offshore the island and are characterized by a thickness less than 2 km and hummocky
terrain in their lower part. Side-slip along a deep decollement are termed slump. Slumps
are slow-moving slope instabilities. The thickness of the deposits can reached 10 km as
the primitive volcano flank is less shattered and disrupted than in the case of a debris
avalanche. The causes of major lateral collapses are still matter of debate, but are
supposed to be related to the magma intrusion in the rift zones in most cases (Denlinger
and Okubo, 1995; Keating and McGuire, 2000).
We present evidence for 36 submarine landslides in Society and Austral islands, French
Polynesia, on the base of a systematic study of submarine island slopes. The detailed
bathymetry of the seafloor around Society and Austral islands were acquired during a
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survey conducted in July 1999 with R/V L’Atalante (ZEPOLYF2, 2001). The in-land
origin of the landslides is deduced from geomorphological analysis of the subaerial
topography of the edifices.
2. Geological setting
Society and Austral islands are located on the south central part of the Pacific plate
(Figure 1). The age of the oceanic crust ranges between 50 and 85 Ma (Mayes et al.,
1990; Munschy et al., 1996). The Society and Austral island chains trend N120º, the
same direction of the present Pacific Plate motion, and are associated with recent
hotspot volcanism. Society island chain extends 750 km from the present hotspot
location over Meetia to three western atolls. It is composed of five atolls and nine
islands. Ages increase from the southeast to the northwest and are reported on figure 1.
Figure 1. Bathymetric map of the Society and Austral islands. FZ stands for fracture zone. Black
stars represent the active areas of Society, Arago and Macdonald hotspots. Ages of the volcanoes are
reported in Ma between parentheses.
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Society Islands exhibits the common evolution stages of the low latitudes oceanic
islands , from elevated island with a growing coral reef to the atoll stage. The Austral
Islands chain is more than 1500 km long and composed of five small islands and one
atoll. Two hotspots are presently active within the Austral alignment, Macdonald
hotspot to the eastern extremity (Norris and Johnson, 1969) and Arago hotspot 120 km
to the southeast of Rurutu island (Bonneville et al., 2002).
3. Landslide distribution
The systematic study of Austral and Society islands slopes, leads us to propose a
classification of the landslide type. The types depend on of the development stage of the
island. Only one example of each type is described with some details herein.
3.1 LANDSLIDE RELATED TO SUBMARINE ACTIVE VOLCANOES
Mouaa Piha (Figure 2) is the shallowest active seamount of the Society hotspot. It is
located to the south-west of Meetia. The upper submarine slopes of Mouaa Piha are
scalloped by three indentations, each of which corresponding to the headwall of
landslides. The lower slopes are characterized by a ripple surface, typical of a debris
avalanche. The deposits are made of small size fragmental material and do not show the
presence of hummocks larger than 100 m, i.e. the spatial resolution of the multibeam
data, except for the north-western landslide where few big blocks are present. The scar
heads range at depth comprised between 500 m for the western landslide, and 1500 m
for the north-western one. The two active seamounts of the Austral Islands, Arago and
Macdonald seamounts present the same landslide characteristics, showing only fine
grained debris avalanche deposits, with very few isolated blocks.
Hence, relative fine grained debris avalanches seem to characterize landslides that occur
on submarine active volcanoes. Blocks forming hummocks are virtually absent. The
morphology of the deposits suggests that the flows were laid down smoothly, and there
is no sign for any cataclysmic events.
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Figure 2. Three-dimensional shaded view of the bathymetry of Mouaa Piha seamount (summit
depth: 143m). Three fine grained debris avalanches (units) can be observed.
3.2 LANDSLIDES RELATED TO YOUNG OCEANIC ISLANDS (< 4 Ma)
Debris avalanches characterize young island landslides. For instance, Meetia, an active
island to the south-eastern boundary of the Society hotspot, has all its flanks mantled by
mass wasting that present a homogenous radial distribution. Basically, submarine slopes
are covered by fine grained deposits as it was the case for active submarine volcanoes,
but large rocky blocks also exist on the north-eastern and western flanks, and deposits of
a debris avalanche with larger blocks can be observed on the southern flank. On Tahiti
Island, a giant landslide of the southern flank (Figure 3) has been dated to the beginning
of the late shield field (0.6-0.8 Ma) (Clouard et al., 2001). This is in good agreement
with Hawaian landslide study of Moore et al (1989), who proposed that main
cataclysmic events happen at the end of the shield building phase.
Raiatea, Tahaa and Bora Bora also present evidence of debris avalanches. In all the
Society Islands except Meetia, there are no trace of fine grained deposits. They could be
covered by subsequent debris avalanche deposits or sediments that occurred on same
areas or because they have been removed by submarine currents .
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Figure 3. Three-dimensional shaded view of the bathymetry and topography of the south of
Tahiti and Moorea islands (after Clouard et al., 2001).
3.3 LANDSLIDES RELATED TO OLDEST OCEANIC ISLANDS (> 5 Ma)
The slopes of the oldest islands of the Austral archipelago, Rapa (5 Ma), Tubuai (9 Ma),
Raivavae (6.5 Ma) and Rimatara (27 Ma but questionable) present evidence of both fine
grained and blocky debris avalanches. If, as the study of the Society landslide suggests,
the trace of fine grained debris avalanches rapidly disappear, then the Austral ones are
recent. They should correspond to erosional processes of old volcanic material between
rift zones, which lead to the stellate characteristics of atolls and guyots (Mitchell, 2001).
3.4 LANDSLIDE RELATED TO TECTONIC EVENTS
Tupai is a small atoll of the Society Islands, north of Bora Bora. Trace of a large
landslide exists on the east side of the edifice (Figure 4). It is characterized by large
debris in the lower part of the slope, their widths reaching several kilometers (3 km x
2.5 km for the greater), and it presents steep toe. These are the characteristics of a debris
avalanche which heads in a giant scarp, reaching 1000 m height and 7 km length. It
might correspond to the scar of the mega blocks. We propose that this landslide is the
result of the collapse where the whole subaerial and submarine eastern flank of the
island slid down to the sea. This explains the non circular shape of the present day coral
reef. An indication of the age of this landslide is given by the sediment coverage of the
deposits: Figure 4 indicates that Bora Bora sediment drifts overlap the southern
hummocks. On another hand, the displacement of the landslide was not linear but
diverted to the north, probably by the submarine volcanic slope of Bora Bora.
Therefore, we can supposed that this collapse occurred at the end of Bora Bora building
stage.
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Figure 4. Three-dimensional shaded view of the bathymetry of Tupai atoll. The average height of
the scarp is 700 m over 7 km wide.
Rurutu Island, in the Austral Islands, has a shape similar to that of Tupai. Rurutu is
characterized by two periods of volcanic activity separated by 11 Ma, and by uplifted
carbonate plateaus. This uplift has been explained by thermal rejuvenation of the
lithosphere due to a second hotspot (Calmant and Cazenave, 1986). A large landslide
exists on the western flank of Rurutu, and its origin corresponds to the western bay.
Unfortunately, data do not cover the lower part of the deposits. However by analogy
with Tupai case, this landslide must correspond to a slump, where the south-western
part of the island disappeared into the sea. The second volcanic episode associated to
vertical motions of the island would be responsible for this slump.
4. Conclusion
Our study reveals 36 major landslides that mantled the submarine slopes of Society and
Austral Islands. These submarine landslides can be classified in three types that are
related to three stages of the edifice life. Submarine active volcanoes are subject to
numerous mass wasting events characterized by small volumes, small size deposits,
visible scars in the upper slopes of the edifices and a radial distribution of the landslides.
During the subaerial volcanic stage, islands are subjects to giant collapses associated
with debris avalanches. This correspond to isolated events, with large subaerial volume
removed. The end of the volcanic activity is marked by the end of giant lateral collapse.
Mass wasting continues, but are mostly of smaller size. These erosional processes
occurred between the rift zones and produced the stellate morphology observed on old
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341
island and on Pacific guyots. In addition, two islands submitted to important vertical
tectonic motions have been the locus of giant flank collapse that re-shape their subaerial
aspect.
5. Acknowledgements
The authors wish to thank Dr Christine Deplus and Jean-François Lenat for their
valuable comments.
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